Proving a Negative 321 



acters or capacities is by way of the germ cells, and more 

 particularly by way of the chromosomal mechanism of the 

 germ cells. Now it does not bring us anywhere to argue 

 that we cannot conceive how a change in bone or muscle, 

 resulting from exercise or from disuse, can reach the chro- 

 mosomes. This helps no more than the argument that we 

 cannot conceive how evolution can take place unless the 

 favorable responses do actually come to be transmitted. It 

 does help, however, to try to find out whether or not germi- 

 nal material is modified, or under what conditions it becomes 

 modified. On this question then there is a great deal of 

 dependable and significant evidence. 



There is a large body of experience in the grafting of 

 plants. In this common horticultural practice the cut base 

 of a twig of one variety of plant (say a plum) is inserted 

 among the living tissues of an older branch of a different 

 variety. The artificial wound is covered and allowed to heal. 

 The scion or engrafted limb continues to manifest the dis- 

 tinct qualities of the tree from which it was taken in leaf, 

 flower, fruit. The stock or the larger plant that now sup- 

 ports the graft also continues to live its life, seemingly un- 

 affected by the intruder. Luther Burbank had at one time 

 a plum tree bearing some 600 distinct grafts; and there 

 was no indication whatever that this close intermingling of 

 tissues affects one or the other member of the union. 



What would seem to be a crucial experiment in animals 

 was performed by Castle and Phillips on guinea pigs of 

 known pedigree (Fig. 86). A female albino was spayed — 

 that is, her ovaries were removed. Then the ovaries of a 

 black guinea pig were engrafted in their place. After the 

 tissues were healed, the white female was mated with an 

 albino male. Let us recall that in these animals albinism or 

 the absence of pigment is recessive, so that two white parents 

 always produce white offspring; and that the dominant 

 character, pigmentation, will appear in the offspring even if 

 only one of the parents is a pure black. In this cross (be- 

 tween an albino male and an albino female carrying ovaries 



